exceeding max rpm

profile grinder · Feb 14, 2016

wondering what you guys might think about a sales rep advising me to ignore the max rpm on an aluminum oxide wheel.

Problem was that in order to stay within the safe rpm range, I'd have to sacrifice surface speed as my wheel diameter dropped. This causes a problem or two because I run a fairly wide range of material, from 408 to pm10v, on only 1 "middle of the road" grade wheel.

sales rep tells me that the max rpm only applies to the wheel at maximum diameter, gives me a max sfpm of 7000 and tells me its perfectly safe to run at that speed throughout the life of the wheel. Everyone else I've asked says he's crazy. any thoughts?

pmtool · Feb 14, 2016

profile grinder said:
wondering what you guys might think about a sales rep advising me to ignore the max rpm on an aluminum oxide wheel.

Problem was that in order to stay within the safe rpm range, I'd have to sacrifice surface speed as my wheel diameter dropped. This causes a problem or two because I run a fairly wide range of material, from 408 to pm10v, on only 1 "middle of the road" grade wheel.

sales rep tells me that the max rpm only applies to the wheel at maximum diameter, gives me a max sfpm of 7000 and tells me its perfectly safe to run at that speed throughout the life of the wheel. Everyone else I've asked says he's crazy. any thoughts?

I think logically he is correct. If your wheel has been worn down to 3 inches from 6 inches you should be able to use it at the rpm suggested for the 3 inch wheel.

I don't know if I would recommend that to someone else or have an employee run it faster, but I would be comfortable doing it myself. If something were to happen I doubt OSHA ignore what is printed on the wheel or in the catalog.

Derek Smalls · Feb 14, 2016

I agree with pmtool. I know we are gonna get flamed by the safety crazy, but it really just takes a little logical thinking and common sense, as said. You can buy the exact same grade wheel in many different diameters. The wheels are NO different, made from the same stuff, same bond, same structure. Only difference is diameter, WHEN NEW, and max RPM rating. So, when a 7" wheel is worn down to 3", it is now EXACTLY the same as a brand new 3" wheel, except it now states the WRONG max RPM on it!

tdmidget · Feb 15, 2016

I put this question to Norton and got the same answer as your salesman. It doers seem counter intuitive at first but the centrifugal force at the peryphery is a result of the surface speed. The problem I see is that some dickhead will overspeed the full size wheel and that is bad. So either train your people to calculate that speed and have a way to measure it or don't let them fuck with the speed.

adama · Feb 15, 2016

Over here we typically take a 9" angle grinder disc and use it till it fits on a 4.5" grinder. Totally against abrasive wheel regs, but in practice its totaly safe, because its the outer diameters that govern speed and the force trying to rip it apart.

Doing this with employees is a gamble though, any time you get to were a individuals safety is dependant only on there common sense it pretty much runs the risk of biting you over here!

Red James · Feb 15, 2016

The centrifugal force trying to explode your wheel varies as the SQUARE of the surface velocity of the wheel. In other words, as your wheel wears down you exert much less force on the o.d. of the wheel. Speed it up (with sense) to maintain the original sfm of the wheel.

Matt_Maguire · Feb 15, 2016

Cylindrical grinders often do this. If you have different size pulleys on the spindle & motor, that's why they are there.

Good luck
Matt

stephen thomas · Feb 15, 2016

I use worn down wheels from the 7" surface grinder on the Cincinati #2 T & C when they get to be less than 5-1/2" dia or so (though at that size I will still be using them on the surface grinder as well for some stuff). When they get really small say (3-1/2" or less) I have even used them on the 5,000 rpm sheave, on the Cincy.

Always ring 'em, whatever the spindle they will hang on.

smt

Peter from Holland · Feb 15, 2016

pmtool said:
I think logically he is correct. If something were to happen I doubt OSHA ignore what is printed on the wheel or in the catalog.

Perhaps or probably OSHA cannot think logically ?????
That is a problem
We have our own problem overhere

Peter from Holland

Matt_Maguire · Feb 15, 2016

peterve said:
Perhaps or probably OSHA cannot think logically ?????
That is a problem
We have our own problem overhere

Peter from Holland

Sad about that... I'm old enough to remember when you got a box of wheels the plain paper washers were tossed in the box with plenty of spares.

Matt

michiganbuck · Feb 15, 2016

I have no problem doing that but the law says the listed is the limit.
I used to take wheels to Mach-B when it was in Warren Mi to have some wheels altered and speed tested, they could speed test and relist the RPM.
If a 7’ wheel was used down to 5” and retested it would likely be safe to re marked to the higher RPM.
But If one told an employee to run higher and it blew to cause injury you would be in Dutch.

We had to list the RPM on every tool room and grinding room spindle according to OSHA.. Or the way we read the script.
We had a box of wheels come in that would not run at the listed.. after 3 wheels blew.. the vendor replaced them and gave us a box of free wheels. Yes they were name brand USA wheels.

profile grinder · Feb 20, 2016

Thanks for all the feedback, everybody. Makes sense to me. no guts no glory, but I've got a 20 inch wheel in my face, so a little second guess now and then makes sense too. haha

pianoman8t8 · Jun 18, 2016

At a Norton (Saint Gobain) grinding seminar, we were told that MAX RPM only applied to the wheel at new diameter dimensions. As your wheel diameter wears down, increase RPM to maintain surface footage. RPM is really irrelevant at that point. It's all about the SFM.

cash · Jun 18, 2016

Agree with the above, the MAX rpm is whent he wheel is new. get your handy SFPM calculator out and as the wheel gets smaller you can increase the speed.

So-lets say your 20" wheel is rated at 1200 rpm. This is about 6,250 RPM. When you wheel is say down at around 14" you could run 1700 rpm and still be safe.

NOW-one thing to make sure-check you machine max operating speed for your spindle. You do not what to toast your bearings.

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